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Unformatted text preview: T Living with Volcanic Risk in the Cascades U.S. Department of the Interior U.S. Geological Survey USGS Fact Sheet 16597 Revised March 2008 U.S. GEOLOGICAL SURVEYREDUCING THE RISK FROM VOLCANO HAZARDS he Cascade Range of the Pacific Northwest has more than a dozen On May 18, 1980, after 2 months of earthquakes and minor eruptions, Mount St. Helens, Washington, exploded in one of the most devastating volcanic eruptions of the 20th century. Although less than 0.1 cubic mile of molten rock (magma) was erupted, 57 people died, and damage exceeded $1 billion. Fortunately, most people in the area were able to evacuate safely before the eruption because public officials had been alerted to the danger by U.S. Geological Survey (USGS) and other scientists. As early as 1975, USGS researchers had warned that Mount St. Helens might soon erupt. Coming more than 60 years after the last eruption in the Cascades (Lassen Peak), the explosion of St. Helens was a spectacular reminder that the millions of residents of the Pacific Northwest share the region with live volcanoes. Volcanoes of the Cascades The volcanoes of the Cascade Range, which stretches from northern California into British Columbia, have produced more than 100 eruptions, most of them explosive, in just the past few thousand years. However, individual Cascade volcanoes can lie dormant for many centuries between eruptions, and the great risk posed by volcanic activity in the region is therefore not always apparent. When Cascade volcanoes do erupt, high-speed avalanches of hot ash and rock potentially active volcanoes. Cascade volcanoes tend to erupt explosively, and on average two eruptions occur per centurythe most recent were at Mount St. Helens, Washington (198086 and 20048), and Lassen Peak, Cali- fornia (191417). To help protect the Pacific Northwests rapidly expand- ing population, USGS scientists at the Cascades Volcano Observatory in Vancouver, Washington, monitor and assess the hazards posed by the regions volcanoes. The more than 1 million residents of the Seattle-Tacoma, Washing- ton, area live in the shadow of 14,411-foot-high Mount Rainier, the (pyroclastic flows), lava flows, and landslides can devastate areas 10 or more miles away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than 50 miles downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to jet aircraft even thousands of miles away....
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